Photovoltaic energy is becoming a very significant power source for several reasons. Fossil fuels are becoming scarcer, and hence more expensive, every day. Furthermore, the burning of fossil fuels releases pollutants and various greenhouse gases which harm the environment. Also, recent events have raised questions as to the safety and cost-effectiveness of nuclear power. For these reasons, traditional energy sources have become far less attractive. Photovoltaic energy, on the other hand, is inherently non-polluting, safe and silent. In addition, recent advances in photovoltaic technology have significantly increased the efficiency, and decreased the cost, of photovoltaic devices.
For example, it is now possible to manufacture large area silicon and/or germanium alloy materials which manifest electrical, optical, chemical, and physical properties equivalent, and in many instances superior to, their single crystalline counterparts. Layers of such alloys can be economically deposited at high speed over relatively large areas and in a variety of stacked configurations. Such alloys readily lend themselves to the manufacture of low cost photovoltaic devices. Examples of particular fluorinated semiconductor alloy materials having significant utility in fabrication of photovoltaic devices are described in U.S. Pat. No. 4,226,898 and U.S. Pat. No. 4,217,374, both invented by Ovshinsky et al, the disclosures of which are incorporated herein by reference.
In a typical large area photovoltaic device, a number of current-collecting structures are employed to convey photo-generated current to a terminal or other collection point. As used herein, these various structures will be referred to as "current-collecting grids" or "gridlines," these terms being understood to include both grids and bus bars as well as any other opaque conductors associated with the light incident side of photovoltaic devices. Current-collecting grids are necessary to withdraw power from the photovoltaic device.
In a photovoltaic device, it is important to maximize the absorption of light incident to maximize the electrical output of the device. The current collecting grids, however, are typically opaque since they are made of high electrical conductivity material such as deposited metal patterns, adhesively adhered metal tapes, metal-containing pastes, metallic inks, or plated layers. The gridlines shade underlying portions of the photovoltaic device, thereby preventing it from absorbing light energy and generating power. Clearly, the gridlines are needed to allow for the efficient withdrawal of photo-generated current, but their presence also detracts from the overall efficiency of the cell. The lines can be made smaller; however, this increases their electrical resistance and thereby tends to decrease cell efficiency. Under the constraints of the prior art, a designer of photovoltaic devices is caught in a dilemma of having to balance the electrical resistance of the cell versus the amount of active area presented for illumination.
In some instances, prior art cells relied upon the use of relatively thin deposits of high conductivity metals such as pure gold, silver, or copper to provide high conductivity, relatively small area gridlines. However, such approaches require the use of sophisticated photolithographic techniques for patterning the gridlines. Additionally, the length of such thin gridlines was limited by the need to avoid high resistivity; consequently, this approach is limited in size and is quite expensive. Lower cost, easier to apply gridlines prepared from paste or ink material are quite desirable; however, they are of lower conductivity and hence must be made fairly thick and wide to achieve sufficient current carrying capabilities. Such materials were not heretofore practical since the gridlines they provide create a high level of shading.
Various attempts have been implemented in the prior art to employ optical systems to concentrate light in areas remote from gridlines. Such approaches involve the use of prismatic arrays and the like. These arrays are typically supported in a spaced-apart relationship with the photovoltaic device or they are adhesively affixed to the light incident side of the device and, when properly aligned, redirect light falling in the region of gridlines to grid-free portions of the device. This technology is typically employed in conjunction with concentrator cells. An overview of this technology is presented by Zhao et al in a paper entitled "Improvements in Silicon Concentrator Cells," published in the Proceedings of the 4th International Photovoltaic Science and Engineering Conference, Sydney, Australia, Feb. 14-17, 1989, Vol. 2, p 581. Use of a Fresnel, lenticular concentrator is also disclosed in U.S. Pat. No. 4,711,972. These prior art approaches employ fairly precise, molded lenses which must either be adhesively affixed to a photovoltaic cell or mounted in a support frame spaced apart from the photovoltaic cell in proper alignment. Use of lenses of this type requires skill in placement and affixation. If the lenses are misaligned, they will be worse than useless since they will direct light to, rather than away from, the gridlines. In use, conditions such as ambient heat and/or mechanical impact can misalign the lens element, thereby decreasing cell efficiency.
It is also important in maximizing the absorption of light to prevent light incident on the photovoltaic device from being reflected away from the photovoltaic device prior to being absorbed therein. Such reflectance can occur either upon initial incidence of the light onto the outer photovoltaic cell surface or after an initial pass through the photovoltaic device. In either event, light energy which reflects outwardly away from the cell is unused and therefore detracts from overall efficiency of the photovoltaic cell.
What is needed is a photovoltaic device having decreased shading from gridlines, which device does not necessitate the precise placement and adhesive affixation of a separate lens element.
Another problem which decreases the efficiency of photovoltaic devices is the reflection of incident light from the surface thereof. A light beam striking the cell will be partially absorbed and partially reflected and the reflected light represents a loss in efficiency; thus it is clearly desirable to minimize reflection as much as possible. Anti-reflective coatings, which comprise transparent layers of varying thicknesses have been used in optical devices for some time to increase the overall absorption of light at a particular wavelength. Such layers are of precise composition and thickness and hence are difficult and expensive to fabricate. Also, anti-reflective coatings operate optimally at particular wavelengths and are not operative to decrease the reflection of broad band illumination.
Another approach to decreasing reflection is disclosed in various of the references mentioned hereinabove and this method comprises affixing a series of light redirecting elements to the upper surface of a cell. As noted in combination with the use of such elements for decreasing gridline shading, their use necessitates alignment and affixation steps and is generally not compatible with the large scale production of low cost, thin film devices.
In accord with another embodiment of the present invention, a series of light redirecting elements may be directly embossed into the front surface of a photovoltaic device so as to increase the light absorbed thereby in a process integral with device manufacture.
The method of the present invention may be adapted for the manufacture of single cells as well as for the manufacture of modules comprised of interconnected cells. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.